2Cromer, R.F. 1970. ‘Children are nice to understand': Surface structure clues for the recovery of a deep structure.' British Journal of Psychology 61:3: 397-408.
3d'Anglejan, A., & G. Tucker. 1975. The acquisition of complex English structures by adult learners. Language Learning 25:281-296.
4Gass, S. 1984. A review of interlanguage syntax: Language transfer and language universals, Language Learning: 34:2.
5Kim, Boomee 1996, Predication in tough-constructions. In The Proceedings of the 14th West Coast Conference on Formal Linguistics,271-285. (ed.) Jose Commacho, Lina Chouerri & Maki Watamabe, Stanford Linguistic Association.
6Mair, Chrisitian, 1990. The predicates embedding infinitival subject clauses, In Studies in English Language Infinitival Complement Clauses in English, 27-92, Cambridge University Press, Cambridge.
7Mathews, S. 1989. The learnability of verbal aspect in L1 and L2. Papers presented at the Language Acquisition Research Symposium.University of Utrecht.
8O'Grady, William. 1997, Syntactic development. London: University of Chicago Press.
9Postal, P.M. & Ross, J.R. 1971. Tough movement si, tough deletion no! Lin 2:544-546.
10Quirk, R., S. Greenbanm, G. Leech, and J. Svartvik. 1972. A grammar of contemporary English. London: Longman.
